Folds a dead key (' ` ^ " ~) plus the following letter into one Latin-9
Key::Char, with the roadmap's `'`+c -> ç case and `'`+space -> literal
apostrophe. Pure and host-tested (12 new tests, 25 total pass).
Not wired into the live decode path yet: it emits non-ASCII and the
editor buffer still assumes byte==char, so wiring waits on the v0.2
UTF-8-correct buffer.
The Key type, US-QWERTY translate, and the edge-detecting boot-report
parser lived in usb_kbd.rs, unreachable by cargo test (the firmware
crate is pinned to the xtensa target). Move them to a dependency-free
#![no_std] + #![forbid(unsafe_code)] crate so the one path that parses
untrusted device bytes can be exercised on the host.
14 tests, including an ASCII-invariant sweep over all 256 usage IDs
(pins the guarantee the editor's byte==char indexing relies on) and a
never-panics fuzz over arbitrary-length/content reports.
